Class Mammalia 561 



extremely specialized in connection with aerial locomotion that they 

 could never have realized all the possibilities of terrestrial living unless 

 they had undergone an anatomic remodeling so profound that the 

 result must have been something other than a bird. At this critical 

 juncture in the history of vertebrates, mammals began to come into 

 prominence. 



Early Mammals 



Mammals, like reptiles, are and always have been mostly ter- 

 restrial. It is therefore not to be expected that quadruped mammals 

 should differ anatomically from reptiles to such an extent as do birds, 

 whose capacity for flight requires drastic and far-reaching modification 

 of the basic vertebrate structure. The characteristics of mammals con- 

 sist of numerous relatively inconspicuous anatomic peculiarities which, 

 however, combine to improve and refine — as compared to reptiles — 

 the animal's adaptation to terrestrial life. Most notable among these 

 are the high and stable temperature of the body, hair, placental vivipa- 

 rous reproduction, and mammary glands. Unfortunately these are all 

 characteristics concerning which fossils yield little or no information. 

 In the case of mammals, the geologic record has produced nothing so 

 spectacular as Archaeopteryx, that halfway stage in the origin of birds. 



Knowledge of the remote past of mammals can be derived only 

 from fossil skeletons and teeth. The skeleton of a modern terrestrial 

 mammal differs from that of a reptile in various particulars, of which 

 some of the most significant pertain to the skull. There is considerable 

 reduction in number of skull-bones, due in part to fusion of some bones 

 which are distinct in reptiles. A secondary or "false" palate is 

 formed by development of a bony partition separating the primitive 

 mouth-cavity into an upper portion, which becomes functionally a 

 backward continuation of the nasal cavities, and a more capacious 

 lower part, which is the definitive oral cavity (Fig. 435). The partition, 

 the "hard palate," arises by ingrowth of thin bony plates from bones 

 of the upper jaw (maxillary and palatine), the corresponding plates 

 from opposite sides meeting and joining in the median plane. This 

 "false" palate, however, is not exclusively characteristic of mammals, 

 for a similar structure occurs in crocodilians. 



The articulation between the skull and the vertebral column in 

 mammals is effected by a pair of occipital condyles, rounded pro- 

 tuberances situated at either side of the foramen magnum through 

 which the spinal cord emerges from the cranium. In all modern reptiles, 

 also in birds, there is a single median condyle ventral to the foramen 

 magnum. Modern amphibians have a pair of condyles, but the earliest 

 known amphibians and reptiles had the single median condyle. 



